I. Field of the Invention
The invention relates to a servo-positioner for a micro-regulating valve especially applicable in flow or pressure-control systems which work at high pressure and/or low flow rates, specifically being a micro-regulating valve, with or without modified seat, actuated by means of a very high-resolution, high-speed servomotor system.
The object of the invention is to provide a quick, precise positioning system for micrometric regulating valves, with an instantaneous response speed for the application thereof in pressure and/or flow rate control systems for micro-pilot or laboratory-scale reactors or equipment.
II. Description of Related Art
Most pilot systems, particularly a reactor for laboratory-scale studies, entail the use of extremely low flow rates and customarily high pressures. This involves working with valves which have considerably low Cv values, e.g., on the order of 10−7 to 10−4. The mechanical building of these valves entails bordering on the precision limits of the machines and tools necessary for building the same. So much so that the manufacturers find themselves forced to resolve the machining of these systems with simple solutions, such as, for example, inserting a cylinder inside a likewise cylindrical hole such that the modulation of the fluid flowing through is not a consequence of the variation in the cross-section of the opening through which it flows, but rather is a function of the length through which it travels in a constantly narrowing cross-section, modulated by the length of the shutter which is inserted inside the seat.
This entails a highly major restriction in the use of these valves, which mainly consists of their very low scalability (difference between the flow rate circulating for a certain pressure differential when the latter is completely closed or completely open), normally on the order of 10-15. This term is normally known as rangeability.
This entails a highly major limitation for these systems, which can be understood by way of the following overly-simplified example: A valve with Cvhypothetical=40 has been selected for a system such that the mean working range thereof is P=5 bar, Q=100 cm3/min, such that Cvhypothetical within this range is 100/5=20. Given that the scalability of this valve will be, due to mechanical building limitations, near 10, the working Cvhypothetical will take on values ranging from 4 up to 40. If the pressure is kept constant at 5 bar and the aim is to study the performance of this process for different flow rates, these flow rates may vary between 200 and 20 cm3/min (a proportion of 10 obviously exists between these values). The problem arises when intending to study the performance of the system for different pressures, due to the fact that, if intended to work at 20 bar with the 20 cm3/min flow rate, a Cvhypothetical of 1 would be necessary, and besides, if it is decided to study the system when the pressure is at 1 bar, in order for the 200 cm3/min to flow through the valve, it would be necessary to have a flow-through capacity or Cvhypothetical of 200.
In this example, that so as to not limit the studying capacity of this process in the prepared system, the “rangeability” of the control valve should have been 200, twenty times greater than the “rangeability” of the micro-flow valves currently on the market.
In the micro-flow systems, controlling the pressure entails a significant problem. In fact, in a catalytic microactivity reactor, the situation which comes to bear when the flow of gases in the system is quite low, on the order of 20 or 50 Ncm3/min and the pressure is high, on the order of 90 bar, is a problem difficult to remedy. The most common alternatives for controlling the pressure in these systems total three in number: the use of a mechanical pressure controller upstream (back-pressure) in an opening-diaphragm-spring mechanism; that of an electronic back-pressure with an MFC valve; and lastly, the pressure in the system can be controlled by closing a control loop, where the end element is a control valve, normally of the pneumatic type, with an electropositioner.
In the mechanical controller, the pressure in the system is transmitted through an opening into the outlet chamber, but this opening is sealed with a diaphragm on which the spring exerts pressure. When the pressure in the system overcomes the pressure exerted on the opening by the spring, the diaphragm opens and the forces then reach equilibrium. The pressure at which this occurs is selected by calibrating the valve or, likewise, by actuating a control which compresses the spring against the diaphragm to select higher or lower triggering pressures.
A device of this type entails a great number of drawbacks: the mechanical control device is of the “only” proportional type, and therefore is affected by an offset error, in other words, once a working pressure has been selected, if the flow rate increases, the pressure value shifts.
In addition to above, although the manufacturers have a vast range of equipment, they do not have instruments suitable for the flow rates within which this type of micro-flow systems work. Hence, when using instruments not suitably dimensioned, the pressure in the system moves the membrane and the fluid flows through the opening, the Cv defining the same is so high that a high majority of the fluid leaks out, the pressure drops and the spring again overcomes the pressure in the system, closing the diaphragm. This operation is repeated when the system once again reaches a pressure high enough to move the diaphragm, the situation in which the diaphragm is in equilibrium at a certain distance from the opening never been achieved. An appropriate data acquisition system could detect a slight oscillation in the system pressure of, for example, ±0.1 bar in 90 bar, which could lead one to think that the control system is quite precise. The situation, to the contrary, is not acceptable for a reaction system, given that the flow circulating through the reactor is pulsating, and the results obtained therefrom are very likely going to be influenced by this characteristic.
Furthermore, these apparatuses are characterized by having a very high dead volume in comparison to the reaction system. If vapors are circulating through the system, they will condense on the inside thereof and will not be evacuated, given the disproportion in the size (the dead volume can be on the order of no less than 25 cm3). Solutions such as that of heating the instruments do not provide satisfactory results in most cases.
The “manual” functioning feature of these systems, which cannot be managed by remote control systems or entail disproportionate costs in doing so, must also not be overlooked.
The electronic back-pressure system with an MFC valve is stable in controlling the pressure, and the field of operativity is relatively acceptable; on the contrary, the main problems arise when there are vapor-phase products in the system. Logically, the proper performance of these instruments is a result of how well their Cv is adjusted to the type of process. This valve flow coefficient is determined by the size of the control valve opening. In a situation in which the vapors present build up to the point of forming a microdroplet of condensate, at the point in time when this microdroplet moves through the opening, the pressure inside the system will rise instantaneously due to the blockage caused by the liquid passing through this micro-opening.
It must also be taken into account that, in these systems, the non-condensates are of widely differing types and that, in terms of the type in question, many of these products can be considered “fouling” in the regard that they can build up on the walls inside the instrument, permanently blocking the control valve opening. In fact, experience goes to show that these instruments are exceedingly delicate for the work in a very high percentage of reaction systems.
Also advantageous to note is that these systems cannot be heated due to their being of the electronic type and due to the valve consisting mainly of an electrically-operated solenoid coil valve.
As an alternative to the above-mentioned systems, the problem of control in these systems can be remedied by configuring a conventional pressure control loop, in which the signal from a pressure transmitter is received by a PID controller which processes the control signal and transmits it to a control valve which acts on the outlet current of the system by modulating the circulating flow and thus controlling the pressure. But, in addition to other problems they may have, the conventional commercial micro-valves are characterized by a low “rangeability” when the required Cv is below the barrier of approximately 10−3. In fact, there are very few manufacturers who supply valves with such low valve flow coefficients.
In the micro-flow valves, which entail Cv values of 10−4 to 10−6, for example, this problem is increased to the limit. Mainly, the low useful working scale of these valves is a result of the problems involved in machining the openings and shutters to the sizes required for generating these low valve flow coefficients. These machining problems lead builders to opt for machining a cylindrical opening and inserting a likewise cylindrical rod through the opening with a certain tolerance between these two elements. Then, the rod runs from its minimum position to maximum run to vary not the cross-section of the narrowest area, but rather the length thereof through which the fluid must pass on flowing through the valve, keeping the flow cross-section constant.